1. Field of the Invention
The present invention relates to a method of manufacturing a crystalline semiconductor film containing silicon that is applied in an active layer of a thin film transistor (hereafter referred to as a TFT), and more particularly, to a spin addition method for a metallic element that has an effect of promoting crystallization. Further, the present invention relates to a semiconductor device having the crystalline semiconductor film.
2. Description of the Related Art
Recently, techniques of forming semiconductor integrated circuits by forming TFTs on an insulating substrate, such as a glass substrate, have been progressed rapidly, and electro-optical devices, typically active matrix liquid crystal display devices, utilizing these techniques have been put into practical use. In particular, active matrix liquid crystal display devices having integrated driver circuits are monolithic liquid crystal display devices in which a pixel matrix circuit and a driver circuit are formed on the same substrate, and the demand for these active matrix liquid crystal display devices has increased along with the demand for making them higher definition. In addition, developments are also advancing toward the realization of system on panels having built-in logic circuits such as γ compensation circuits, memory circuits, and clock generator circuits or the like.
However, it is necessary that driver circuits and logic circuits operate at high speed, and therefore the application of amorphous silicon films to the active layers, which form regions such as channel forming regions, source regions and drain regions in TFTs, is unsuitable. TFTs having a polycrystalline silicon film as an active layer are coming into the mainstream at present. The application of low-cost glass substrates as substrates for forming TFTs is demanded, and the development of processes capable of being applied to glass substrates is flourishing.
For example, a technique is known in which a metallic element having a crystallization promotion effect, such as Ni (nickel) (hereafter referred to simply as a catalyst element) is introduced into an amorphous silicon film, and then a crystalline silicon film is formed by heat treatment. It is clear that crystallization is possible by heat treatment if a temperature on the order of 550 to 600° C., less than the heat resistant temperature of the glass substrate, is used as the heat treatment temperature. It is necessary that the catalyst element be introduced into the amorphous silicon film with this crystallization technique. Methods such as plasma CVD, sputtering, evaporation, and spin addition can be given as introduction methods.
A spin addition method, in which a solution containing a catalyst element (hereafter referred to as a catalyst element solution) is added by spinning, is disclosed in JP 07-211636 A as a method of efficiently introducing a catalyst element into the vicinity of the surface of an amorphous silicon film. The spin addition method for the catalyst element solution as disclosed in the aforementioned unexamined patent application publication has the following characteristics:
(Characteristic 1) The amount of the catalyst element added to the surface of the amorphous silicon film can easily be controlled by controlling the concentration of the catalyst element within the catalyst element solution;
(Characteristic 2) The minimum amount of the catalyst element required in crystallization can therefore be added easily to the surface of the amorphous silicon film; and
(Characteristic 3) It is necessary to reduce the amount of the catalyst element within the crystallized crystalline silicon film as much as possible for reliability and electrical stability of the semiconductor device. The smallest amount of the catalyst element necessary for crystallization can be easily added by regulating the catalyst element concentration of the catalyst element solution with the spin addition method, and therefore the introduction of an excess amount of the catalyst element can be suppressed, which is advantageous for reliability and electrical stability of the semiconductor device.
The size of the glass substrates used in manufacturing of liquid crystal display devices has been becoming larger in view of the goal of applications to large size screens and increasing productivity. It has been projected that in the future, glass substrates that exceed 1 m on a side will be in use.
The above stated spin addition method for the catalyst element is one in which a liquid builds up on the substrate by dripping the catalyst element solution down onto the substrate surface, and the catalyst element solution that has been dripped down is then spun off by rotating the substrate at high velocity, thus adding a desired amount of the catalyst element to the substrate surface. This spin addition method is characterized in that the amount of the catalyst element added to the surface of the substrate can be easily controlled, and the like, and therefore it is a very important technique that is currently undergoing consideration for being put into practical use. However, there is a problem in that the uniformity of the amount of added catalyst element becomes poor as the substrate size becomes larger. In particular, the non-uniformity becomes a problem that cannot be ignored when the diagonal length of the square substrate is equal to or larger than 500 mm.
The main reason that the uniformity becomes poor is thought to be because at the spin drying state after the catalyst element solution has been applied to the substrate, the relative motion velocity with respect to air between the central portion of the substrate and regions in the periphery of the substrate differ. Caused by this, the evaporation speed of solvent components of the catalyst element solution varies within the surface of the substrate, and as a result, drying unevenness develop between the central portion and the peripheral regions.
FIG. 3 is a diagram showing the relationship between the size of the square substrate and the motion velocity at the edge portions of the substrate, and the following can be considered as causes of the generation of drying unevenness. For example, if the catalyst element solution is added to a 250 mm square substrate by spin addition, the motion velocity of the central portion of the substrate with respect to air is 0 m/min when the rotational velocity is 500 rpm (500 rotations/minute), while the edge portions of the substrate rotate at a motion velocity of approximately 400 m/min. Motion with respect to air thus becomes higher speed with increasing distance from the central portion of the substrate, and therefore friction with the air becomes severe, and the solvent components of the catalyst element solution evaporated very rapidly. Drying unevenness therefore develop due to the differences in evaporation speed of the solvent components between the central portion of the substrate and the edge portions of the substrate.
In addition, the drying unevenness caused by the different drying speeds of the solvent components tend to manifest at corner regions of the square substrate. It is thought that this is because air is pushed aside along with rotational motion in the corner regions of the substrate, and therefore the friction with the air becomes exceptionally severe there. These types of drying unevenness are large problems that influence the amount of deposited catalyst element, and that influence various fluctuations, such as fluctuations in the final crystallization ratio, the size of crystal grains, and their alignment after crystallization.